1. Field of the Invention
The present invention relates to the field of locks, and more particularly to entry door lock sets.
2. Prior Art
In recent years there has been a steadily increasing trend toward using dead bolts on entry doors which operate from both sides with a key. Such locks have the advantage of higher security when the inner key has been removed, as mere manual access to the interior lock portion will not allow an intruder to withdraw the bolt. This is particularly advantageous when used on doors having window panels or the like, as manual access to the interior lock portion is particularly simple in such instances.
Dead bolts which are only operable from either side by a key have certain disadvantages however. If the dead bolt is locked and the key removed while persons are still within the enclosure protected by the dead bolt, exiting in emergency situations can require precious seconds to locate the key and actuate the dead bolt, or could even be prevented if the key were not immediately present. The situation is particularly severe in emergencies such as fire, in that the attendant panic may prevent a person from using the key to unlock the dead bolt even if the key were only a few feet away.
One technique which has been proposed to at least minimize such risks is to provide a special key and plug, used in conjunction with a special central friction shoe in the lock assembly to confine the key in the interior lock if the dead bolt has been extended by that key (i.e. the dead bolt locked from within) but to allow removal of the key from the interior lock if the last operation of the interior lock was to retract the dead bolt. (See U.S. Pat. No. 4,028,917). Such a system is effective to prevent a person locking a dead bolt from within the protected enclosure from thereafter removing the key, so that the key is always in position in the This preserves the rapid exit capability when the protected enclosure is occupied and the risk of burglary is relatively low, but allows the removal of the interior key and thus provides maximum security when the protected enclosures is unoccupied and the risk of burglary is highest. (It should be noted that this concept is not absolutely foolproof, in that if the last operation of the interior lock is to withdraw the dead bolt, the interior key can be removed. Thereafter, if the dead bolt is locked (extended) using the exterior lock while people remain within the enclosure protected by the lock, the interior key will not be in position during a panic situation. This, however, normally would be the exception rather than the rule, whereas without such a provision the interior key may be indiscriminately removed at any time.)
The lock of U.S. Pat. No. 4,028,917 has a plug which in addition to the standard key slot is also slotted in a plane perpendicular to its axis, the slot extending from a position directly opposite the key slot and being slotted through over half its cross-section. In addition, a peripheral groove of some substantial depth is cut in the plug in axial alignment with the slot so as to intersect the edge of the key slot. A spring clip generally in the form of an arc segment of substantially greater than 180.degree. is located in the slot of the plug, with the legs thereof extending into the peripheral groove so as to be rotatable with respect thereto to intersect the edge of the key slot. This, in conjunction with a cooperatively disposed slot on the back edge of the blade of the key, provides for the confinement of the key dependent upon the position of the spring member. A tab on the spring member in cooperation with the slot provides a lost motion between the plug and the spring member so that the spring member will retain the key when used to extend the dead bolt and will allow the withdrawal of the key after being used to retract the dead bolt.
The foregoing prior art system has certain advantages in that it achieves the desired result with a relatively simple mechanism. It has certain disadvantages however including the fact that the plug is grossly weakened and that it requires a special key having an appropriately disposed slot therein.